This invention relates to mechanism for disconnecting an elevated pickup unit component of lifting apparatus from a tilt-up concrete wall slab in raised position following a lifting operation, the lifting apparatus also including an insert embedded in the slab and the pickup unit being connected to the slab by fastening means engaging the insert. More particularly, the invention relates to mechanism including a flexible fastener coupling tool which may be operated remotely for disengaging such fastening means from the insert when the fastening means is disposed at a location above and remote from an operator of the mechanism.
Tilt-up construction is a job-site form of precast concrete construction. It involves prefabricating concrete wall slabs or panels flat on either the building floor slab or a temporary casting slab, then lifting or "tilting" them up with a mobile crane and carrying them to their final locations, where they are installed as vertical walls and become integral parts of the completed structure. In the erection of a tilt-up wall slab, a concrete slab is connected temporarily to lifting or hoisting equipment, utilizing accessory lifting apparatus connected to the slab. The lifting apparatus includes an anchor insert which is embedded permanently in the slab, and a pickup unit which is releasably connected to the insert and thereby the slab, by fastening means engaging the insert. The pickup unit also is releasably connected to the lifting equipment. In use, a plurality of inserts is embedded in a concrete slab by pouring wet concrete therearound and setting the concrete, and a pickup unit is connected to each embedded insert and also to the lifting equipment. The lifting equipment is operated to raise the slab from a horizontal position in which it is cast, to a vertical position in which it serves as a building wall. The wall slab is braced, the pickup units are disconnected from the slab, by disengagement of the fastening means from the inserts, and the wall slab is integrated into the building structure. The inserts remain embedded in the slab, and the pickup units may be used repeatedly with inserts embedded in additional wall slabs. Lifting apparatus for a tilt-up concrete wall slab is illustrated in U.S. Pat. Nos. 3,431,012 and 2,794,336, the latter also illustrating the manner in which the lifting operation is accomplished.
The pickup units to be disconnected after a wall slab has been raised by the tilt-up construction method are in elevated positions on the face of the slab. It is necessary for workmen to scale the slab in some manner for disconnecting the units, usually on ladders placed against the wall slab and supported by the floor slab. One man climbs the ladder, and another man is required to hold the ladder. The man on the ladder must reach out and loosen and/or remove a fastening member, and it may be necessary to pull out a pickup unit weighing over 20 pounds. Meanwhile, the workmen are working beneath a crane and its lifting equipment. The pickup units are lowered to the ground by the crane cables.
The foregoing method of removing the pickup units has been used for many years. However, risks are involved when the men are disconnecting the units while standing on ladders and working beneath the crane rigging, and proportionate expenditures of manpower and time are required. It would be highly advantageous to eliminate the need for conducting such operations from ladders or other elevated equipment, by providing remotely operable mechanism for disconnecting the pickup units, enabling the units to be removed by workmen remaining at ground level.